The short prequel tells how Ajax Penumbra ends up at the bookstore that will come to bear his name and of his introduction to the mysterious society behind the store. A young Penumbra, working for the college he recently attended, is searching for a rare book, now lost, that was last known to be in San Francisco more than a century ago. He is doing this at the behest of a professor of Occult Lit, a secret series of classes he stumbled into because he very intentionally signed up for a class on sentence diagramming. But once in the course, he finds the work fascinating and continues plumbing the mysteries in it until after graduation he lands the job in the school's library that sends him on his literary quest to San Francisco.
In California, he meets several people who will figure prominently in Mr. Penumbra's 24-Hour Bookstore and the reader has a chance to see what his relationships with everyone were from the beginning. Penumbra also maintains a connection with his old roommate at tiny Galvanic College, Claude Novak, a science fiction fanatic and computer programmer in the nascent computer industry in Silicon Valley. With the help of his new friends and his old friend Claude, he will dig into history helped by cutting edge technology and his journey into the mysteries of the 24-hour bookshop will commence.
Definitely written for those who have already enjoyed the longer novel, this is a fun addition to the world Sloan has created. It is interesting to see how Mr. Penumbra got his start in San Francisco and what drove him as a young man. As a tale, it is enigmatic enough that it probably doesn't stand up too well without prior knowledge of the novel but as an additional treat to tack onto the magic of the novel, it is worth a read.